Method of mounting electrode

ABSTRACT

A method of cathodically protecting a ferrous metal object against corrosion in an electrolyte. A magnet of high dielectric ceramic material has a protecting electrode mounted thereon and is placed with the electrode thereon against the object to be protected with the magnet directly against the object. The electrode is electrically connected to the object to be protected through a path outside the magnet. The electrode is secured tightly to the object by the magnet and it also acts to electrically insulate the electrode from the object to be protected.

United States Patent Beer et al.

151 3,657,084 [4 1 *Apr. 18, 1972 [54] METHOD OF MOUNTING ELECTRODE [72]Inventors: Ernst Beer, Zorgvlietstraat 176, The Hague, Netherlands;Henri Bernard Beer, Vogelengangstraat 33, Kalmthout, Belgi- Notice: Theportion of the term of this patent subsequent to May 19, 1988, has beendisclaimed.

22 Filed: Dec. 3, 1969 [21] Appl.No.: 881,767

Related US. Application Data [63] Continuation of Ser. No. 665,178,Sept. 1, 1967, Pat. No. 3,513,082, which is a continuation of Ser. No.341,003, Jan. 29, 1972, abandoned.

[52] US. Cl ..204/147, 204/148, 204/196, 204/197 [51] Int. Cl. ..C23f13/00 [58] Field ofSearch ..204/147, 148, 196, 197, 286, 204/297;248/206 A [5 6] References Cited UNITED STATES PATENTS 2,752,308 6/1956Andrus ..204/196 2,934,485 4/1960 Sabins ..204/197 2,954,257 9/ 1960Besuch ..248/206 A 3,012,959 12/1961 Banard ..204/197 3,133,873 5/1964Miller et al.. .....204/196 3,513,082 5/1970 Beer et a1 ..204/197 OTHERPUBLICATIONS Justin, Official Gazette" Vol. 659, p. 590 Oct. 6, 1952Primary Examiner--T. Jung AttorneyWenderoth, Lind & Ponack [57] ABSTRACTA method of cathodically protecting a ferrous metal object againstcorrosion in an electrolyte. A magnet of high dielectric ceramicmaterial has a protecting electrode mounted thereon and is placed withthe electrode thereon against the object to be protected with the magnetdirectly against the object. The electrode is electrically connected tothe object to be protected through a path outside the magnet. Theelectrode is secured tightly to the object by the magnet and it alsoacts to electrically insulate the electrode from the object to beprotected.

4 Claims, 6 Drawing Figures METHOD OF MOUNTING ELECTRODE Thisapplication is a continuation of application Ser. No. 665,178, filedSept. 1, 1967, now U.S. Pat. No. 3,513,082, which in turn is acontinuation of application Ser. No. 341,003, filed Jan. 29, 1964, nowabandoned.

The present invention relates to an electrode system for protecting ametal object against corrosion in an electrolyte.

The invention relates particularly to electrodes of zinc, aluminium,magnesium, platinum, platinum-coated titanium, which are magneticallysecured to ferro-magnetic metals, such as steel, steel compounds,nickel, cobalt, for protecting these metals cathodically againstcorrosion in electrolytes, such as sea-water, brackish water,ground-water, cooling-water, boiler-water and the like.

The cathodic protection of ferromagnetic metals by means of electrodesin inter alia known from Netherlands Pat. Nos. 36,564, 72,206, 74,279and 81,577, German Pat. No. 644,418, U.S. Pat. Nos. 2,766,200, 2,863,819and 3,011,959, French Pat. No. 1,271,669 and British Pat. No. 870,086.

It is known to fasten all sorts of articles to a ferromagnetic base bymeans of magnets.

Dutch Pat. No. 100,332 discloses the mounting of an electrode by meansof a metal magnet, for example, to a steel object protected againstcorrosion in an electrolyte. The electrode used according to this patentis a sacrificing anode, which is secured to the object to be protectedby means of a metal magnet, and is electrically connected to the objectto be protected inter alia by means of the metal magnet.

By securing the sacrificing anode to the object to be protected by meansof a metal magnet, the replacement and/or renewal of the anode and ofthe metal magnet is a simple operation, while it is not necessary toequip the wall to be protected of the object with special fasteningmeans.

It has been found, however, that it is not so simple to secure asacrificing anode to a metal magnet, and that the fitting of thesacrificing anode to the object to be protectedby means of this metalmagnet is not satisfactory in all cases.

Such an arrangement has many disadvantages, and the problems inherent tothe cathodic protection are mostly not solved or only partly so.

The disadvantages of the known method of securing an anode by means of ametal magnet, are inter alia the following.

1. Hitherto only metal magnets have been used for this purpose, andthese especially in the horseshoe form. These metal magnets aregenerally made of a metal alloy consisting of steel, nickel, cobalt,aluminium etc. If the electric potential of such a metal magnet ismeasured in sea-water relative to ships steel, there appears to be animportant difference in potential in favour of the magnet or in favourof the ships steel to be protected, by which strongly corrosive effectsare generated. This corrosion-promoting potential of these metal magnetsmay sometimes be 220 millivolts and more relative to the object to beprotected so that, if through unforeseen conditions the anode does notfunction or insufficiently so, this metal magnet may locally effectstrongly corrosive phenomena, which is very detrimental to the metalmagnet or to the object to be protected, e.g., ships hulls, tank-wallsetc.

2. The metal magnets of high alloy steel referred to above are used withsacrificing anodes because these metal magnets effect the directelectric contact with the object to be protected. To this effect, it isnaturally desirable that there is maintained an electric transitionresistance as low as possible, for which reason the pole shoes and theobject to be protected should be kept free of oxides etc. It would beexpected that such an electric contact could be very well maintained solong as the sacrificing anode is secured to the magnet and the poleshoes of the metal magnet are in contact with the object to beprotected. Contrary to this expectation, practice has shown thatcorrosion phenomena occur between these pole shoes and the object to beprotected, so that the electric conduction becomes inadequate and theattraction of the metal magnet relative to the object is considerablyreduced. This phenomenon is to be ascribed to the circumstance that,however smooth the pole shoes of the metal magnet and the surface onwhich the metal magnet is mounted may be, there will still always be awater film between them, and a greater amount in the ever-presentscratches and roughnesses. Although the sacrificing anode will supplyits protective energy, this anode cannot sufficiently exert itsinfluence in the interspace between the pole shoes and the surface to beprotected, because the potential difference between the pole shoes ofthe metal magnet and the ships steel locally inhibits the cathodicprotection, so that corrosion occurs. This corrosion may after some timebe such that pit-corrosion occurs in one of the two contacting surfaces,the pits being in the long run filled with rust, so that the electricconduction becomes insufficient and the magnetic attraction is reduced.As a consequence, the object is under-protected and further corrosionoccurs, which may grow to such an extent that the anode and the metalmagnet comes ofi the object and the cathodic protection stops entirely.

3. 1n the high-alloy metal magnet, selective self-corrosion may occurwhich renders it unfit for use, so that one of the attractions of thismanner of securing, namely, an economic and continuous fixation, islost.

4. The existing metal magnets are very difiicult to process, and, forexample, are not easily made in a compact and streamline form. Horseshoemagnets create cumbersome vortices at some velocity of the ship, and,owing to their form, they are as it were forced off the ships hull.Streamline magnets are very desirable, it is true, for use upon theships hull, because they offer little resistance in the navigation, butthey are difficult to make because high-alloy metal magnets are noteasily processed and made compact.

5. The existing metal magnet are difficult to make or to assemble intounits having a high magnetic power, so that only electrodes ofcomparitively small size and low weight can be used, which excludestheir use in ships of some size.

6. The existing high-alloy metal magnets have the property that inmounting, when they are dropped or struck against other objects, theystrike sparks, so that their use in tirehazardous spaces (e.g. oiltankers) must be considered impossible.

7. The storage of the known metal magnets must be regarded as verydifficult, because, if the magnetic poles are not closed, the magnetssoon lose their magnetism. Shutting off the magnet poles when theelectrode is already mounted to the magnet, presents difficulties inpractice for the storage.

8. The very high-alloy steel magnets are very difficult to process andas a consequence, a good form of mounting for sacrificing anodes of somesize is very difficult to find.

9. Experiments have shown that the existing metal magnets rapidly losetheir magnetic power when they are subjected to vibrations as occur forexample in a ship.

10. The method of securing electrodes by means of metal magnets can onlybe used for mounting sacrificing anodes, because in this case thecurrent is directly passed from the sacrificing anode to the surface tobe protected. In the cathodic protection by the impressed voltagetechnique, the fixation by means of a metal magnet could only be used ifit would be possible for the anode to be mounted to the metal magnet inelectrically insulated condition, which requirement, however, canhardly, if at all, be met.

It is an object of the present invention to remove the disadvantages ofthe prior method of securing electrodes by means of metal magnets, andto provide a magnetic fixation of electrodes which can be successfullyused in almost all cases.

To this end, according to the invention, use is made of an electrodesystem comprising at least one magnet of a material which iscorrosion-resistant in the electrolyte.

According to the invention, the electrode system comprises at least onemagnet of a material which is electrically conductlve.

Magnet units of this ceramic material are here understood as beingmagnets consisting of mixtures of oxides, such as e.g. iron-bariumoxides, iron-lead oxides, which are well mixed and subsequently sinteredto mechanically very strong bodies and then after magnetization formmagnets of exceptional strength and life. These ceramic magnets are notelectrically conductive, possess no potential relative to the metal theyare to protect, have a resistance of about 100,000 ohms per cm. and moreand are further entirely resistant to corrosive electrolytes.

Although such ceramic magnets have been made in large quantities for allsorts of purposes since 1946, it is, according to the invention, for thefirst time proposed to use the exceptionally good properties of thesemagnets for mounting electrodes on surfaces to be protected. Thefarourable properties of these ceramic magnets for the purpose accordingto the invention may be summarized as follows:

1. They do not possess a potential relative to the surfaces to beprotected so that there can never occur an activated corrosion around orunder the magnet.

2. The ceramic magnets allow of being constructed in combination withelectrically conductive members permanently maintaining the idealcontact with the object to be protected, and which do not have anyadverse difference in potential relative to the object to be protected.

3. The magnetic attraction of these ceramic magnets is ensured forscores of years both in open and in closed condition, so that a stableattachment to the object to be protected is obtained.

4. The magnetic attraction is not reduced by the vibrations of, forexample a ship, because these very ceramic magnets are particularlystrongly resistant to vibrations and to other forms of mechanical loads.

5. Since no corrosion can occur between the ceramic magnet and theobject to be protected, no magnetic losses can arise as a result ofthis, and undesirably reduce the attraction of the ceramic magnets tothe object, so that the magnet will not come off, with all adverseeffects of this.

6. The ceramic magnets allow of being made in all possible shapes, andof being assembled from a plurality of small units to one large wholehaving a very strong magnetic attraction, should this be desired. Theceramic material is easily processed, so that the necessary streamlineform can be made. In the case of, for example, navigating vesselscathodically protected in this manner by means of an electrode systemaccording to the invention, unnecessary resistance in the water isprevented, and the magnet, too, will not encounter extra loads in viewof which the capacity of the magnet should be made unduly high.

7. When they are dropped, struck or subjected to other impact contactswith rusty iron, the ceramic magnets have no tendency to strike sparks,so that fire hazard in, for example, tankers is prevented.

8. By virtue of the structural possibilities with ceramic magnets andthe possibility of assembling strong magnet systems from smaller unitsof these magnets and to shape the same as desired with easily processedmetal capable of forming a good anchorage for the electrode, a strongelectrode-magnet system is obtained, which provides an excellentcathodic protection, is easily mounted and can be economically replaced.

9. The magnetic power of the ceramic magnets can be reinforcedconsiderably by a combination with other ferromagnetic materials which,as regards their potential are not detrimental to the surfaces to beprotected either.

10. The combination of ceramic magnets per se, or in conjunction withferromagnetic materials of the same lower potential than the object tobe protected, a good magnetic adherence and proper electric contact isafter a short time even obtained on a rusty base. This is caused by thefact that rust, which consists of a loose mixture of Fe O and Fe OH, bythe contact with the system according to the invention and under theinfluence of the magnetic force thereof, is in an electrolyte-like, forexample, sea-water, after a short time converted into stronglyferromagnetic Fe O which adheres well to the metal base and is moreovera good electric conductor. No other form of attachment but a magneticone could effect this reaction.

According to the invention one magnet unit may have the form of a flatdisc.

According to the invention, the disc-shaped magnet unit may be providedwith a central hole.

Further, according to the invention the central hole in the disc-shapedmagnet unit may be threaded.

According to the invention one magnet unit may be sandwiched betweenferromagnetic members which direct, bundle and/or reinforce the magneticpower of the magnet units.

Furthermore, according to the invention, the areas of the ferromagneticmembers having the magnet units between them may be in excess of theareas by which the magnet units is in contact with the ferromagneticmembers.

Furthermore, according to the invention, the magnet unit and theferromagnetic members may be kept together by means of electricallyconductive fasteners.

According to the invention, the fasteners may be spaced from the magnetunits.

Furthermore, according to the invention, a plurality of magnet units maybe combined to a magnet system, each pair of adjacent magnet unitshaving a ferromagnetic member between them.

Furthermore, according to the invention, all the electrically conductivecomponents of the magnet system which are in direct contact with theobject to be protected cathodically may have an equal or more negativepotential than the potential of this object.

According to the invention, the threaded opening of the ceramic magnetunit may fit a pin-shaped, threaded projection of a reinforcing frame ofan electrode, the thickness of the disc-shaped ceramic magnet unit beingin excess of the length of the threaded portion of said pin-shapedprojection.

According to the invention, at least one of the ferromagnetic members ofthe magnet system composed of ceramic magnet units and ferromagneticmembers may be provided with a threaded opening fitting a pin-shaped,threaded projection of a reinforcing frame of the electrode.

According to the invention, the fastener for keeping the system ofceramic magnet units and ferromagnetic members together may be providedwith threaded openings, by means of which the electrode can be connectedwith the fastener.

According to the invention, the magnet system may be accommodated in aspace in the preferably streamlined electrode.

According to the invention, the magnet system may be surrounded bysynthetic material except for the pole surfaces.

According to the invention, the electrode and the magnet system mayextend in alignment with each other and be separated by an insulatingscreen.

The invention will be further explained with reference to the drawings,illustrating, by way of example, some embodiments of the invention. Inthe drawings:

FIGS. 1 and la show diagrammatically the use of an electrode systemaccording to the invention, the electrode being a sacrificing anode;

FIG. 2 shows diagrammatically the use of the electrode according to theinvention for the cathodic protection by means of the impressed voltagetechnique;

FIGS. 3 and 3a show an embodiment of an electrode system resulting in alarge cathodic spread; and

FIG. 4 shows diagrammatically an embodiment of an electrode systemaccording to the invention having a streamlined electrode.

Referring to FIG. 1, the steel ships hull l is cathodically protected bya sacrificing electrode 2, which is fastened to the ships hull by meansof the magnet unit 3.

According to FIG. 1, the electrode system consists of a sacrificinganode 2 and a ceramic magnet unit 3. The sacrificing anode is reinforcedby reinforcing bar 4 embedded in the material of the anode to projectfrom the material of the anode by an end S, which is provided with ascrew thread.

The ceramic magnet unit 3 has the form of a fiat disc (FIG. 1a), whichis magnetized in the direction of its thickness. The

disc 3 has a central threaded opening 6 fitting the threaded end 5 ofthe reinforcing bar 4. The threaded length of the end 5 is shorter thanthe thickness of the disc 3, so that the terminal face 7 of thereinforcing bar does not project from the opening of the disc and mayextend into the opening 6, for example, up to halfway the thickness ofthe disc 3. The end 5 of the reinforcing bar is provided with a contactterminal 8. The electrode system is mounted on the surface of a body 1to be protected against corrosion, such as the steel hull of a ship, andis located below the level 51 of the seawater.

The anode 2, which consists, for example, of magnesium, is connected byway of the reinforcing bar 4, which acts as a conductor, the contactterminal 8, conductor 9, measuring instrument 10, variable resistor 11to the hull of the ship 1. By means of the measuring instrument theoperation of the apparatus can be checked, and by varying the resistor11, it can be varied and adjusted.

This construction can very well be used for protecting the painted hullof laid-up ships, the sacrificing electrode preferably consisting ofmagnesium. The ceramic magnet forms in this respect an ideal possibilityof attachment, because magnesium has so high a potential relative tosteel that over protection and damage to the paint would occur, if themagnesium electrode should be mounted on the hull of the ship by meansof a metal magnet. Since, according to the invention, use is made of aceramic magnet, the electric connection of the sacrificing electrode tothe ships hull is effected by way of the conductor 9 connected to thereinforcing bar 4 and the variable resistor 11, which presents apossibility for adjustment.

FIG. 2 shows an embodiment of an electrode system for cathodicprotection by means of the impressed voltage technique. The electrodesystem consists of the ceramic magnet unit 13, which is disc-shaped andstreamlined, and provided with a threaded opening 15, similarly to theembodiment of the ceramic magnet shown in FIG. 1. The electrode 14 islikewise streamlined and may consist of an inert material, such as forexample, platinum, platinum-coated titanium, rhodinated tantalum. Thepin-shaped threaded protection 16 of the electrode 14 is screwed in theopening 15 of the magnet unit. The projection 16 has a contact terminal17. The threaded length of the projection is shorter than the thicknessof the disc-shaped ceramic unit 13, so that the end does not projectfrom the opening 15 of the unit. The electrode system is mounted on thesurface of the object 12 to be protected against corrosion below thelevel 52 of an electrolyte. The electrode 14 is connected by means ofthe conductor 18 connected to the contact terminal 17 to the positiveterminal of a cource of direct current 19, the negative terminal of thissource of direct current being connected via conductor 20 to the object12 to be protected. In the same manner as according to FIG. 1, avariable resistor and a measuring instrument may be accommodated in theconductor 18 according to FIG. 2. The ceramic unit 13 performs thefunctions of fastening the electrode 14 to the object 12 to be protectedand electrically insulating the electrode 14 from the object.

FIG. 3 shows an embodiment of an electrode system according to theinvention for mounting an electrode on the inner walls of tankers forcarrying, for example, petroleum, or filled with sea-water as ballast.

The magnet system in this embodiment comprises twoparallelepipedon-shaped ceramic magnet units 21 and 22 which enclose anauxiliary member or pole piece 23 of ferromagnetic material and arethemselves enclosed by the ferromagnetic auxiliary members 24v and 25which are pole pieces which function as pole shoes. The flat ceramicmagnet units are magnetized in the direction of their thickness, the twoequal poles of the ceramic magnet units being directed towards eachother and away from each other, respectively. The auxiliary members,which consist of ferromagnetic material which has good electricalconductivity, have their edges projecting from the enclosed ceramicmagnet units, the

whole being kept together by the bolts 26 and 27, which at the same timeserve as electrically conductive connections between the auxiliarymembers. As appears from FIG. 3a, which shows a cross-section on theline Illa Illa in FIG. 3, the bolts 26 and 27 are spaced from theceramic magnet units to prevent the magnetic circuit to be closed by wayof these bolts.

For further preventing the closure of the magnetic circuit via the bolts26 and 27, these bolts may be made of non-ferrous metal. The auxiliarymembers are in contact with the surface 28 to be protected, and theedges by which they project from the enclosed ceramic magnet units maybe suitably adapted to the form of the surface to be protected, e.g. thehull of a ship. In this connection, the edges may be of the same shapeas the surface to be protected they are in contact with, or,alternatively, the extent to which they project beyond the edges of theceramic magnet units may be varied according to the shape of the surfaceto be protected. The intermediate auxiliary member 23, which is thickerthan the other auxiliary members 24 and 25, is provided with a threadedopening 29, into which the threaded pinshaped projection 30 of areinforcing frame 31 is screwed. The pin-shaped projection 30 isprovided with a contact terminal 53. A screen 33 of insulating material,fastened to the magnet system simultaneously with the electrode 30 isfor obtaining a greater spread of the cathodic effect of the electrode32. The whole magnet system, except for the pole surfaces of theauxiliary members in contact with the surface 28 to be protected, issurrounded by insulating material 34. In this connection, the materialof the auxiliary members 23, 24, 25 in contact with the surface to beprotected is preferably of the same potential in the electromotiveseries as the potential of the material of the surface of the object tobe protected, so that, if the electrode material 32 should no longer bethere, no corrosion-inciting potential can be created between the magnetsystem and the surface to be protected.

This embodiment may be extended by more ceramic magnet units andferromagnetic auxiliary members practically without limits, so that avery powerful magnetic system is obtained. According to FIG. 3, theferromagnetic auxiliary members 23, 24 and 25 are all electricallyconductively interconnected by the bolts 26 and 27 which keep the entiremagnet system together. In this connection, the auxiliary members forman electrically conductive connection between the electrode, which maybe a sacrificing anode, and the surface of the object 28 to beprotected. It is possible, however, for example, by inserting a thinpertinax plate between the magnet system and the surface to be protectedto have the electrode system electrically insulated from the surface tobe protected, and, by means of the contact terminal 53, to connect theelectrode system as illustrated in FIGS. 1 and 2. The electrode systemmay be electrically insulated from the wall to be protected in adifferent manner by increasing the openings in the auxiliary member 23in accordance with the dotted lines 56 in such a manner that the bolts26 and 27 do not touch this auxiliary member and by further shifting theauxiliary member 23 over a small distance to the rightaccording to thedotted line 54 in FIG. 3, so that this auxiliary member 23, to which theelectrode 32 is fastened, does not come into contact with the surface tobe protected. In the case that the magnet system is composed of aplurality of ceramic magnet units, at least more than two, a very shortdistance between one or more auxiliary members and the'surface to beprotected has little influence on the total attraction exerted by theceramic magnet system.

In the embodiment according to FIG. 4, the electrode system comprisestwo ceramic magnet systems 35 and 36, which are accommodated in therecesses 37 and 38 of a streamline electrode 39. The ceramic magnetsystem 35, like the magnet system 36, not shown, is composed ofdisc-shaped ceramic magnet units 40, 41, which have a central opening42. The ceramic magnet units are stacked with insertion of aferromagnetic member 43 and are sandwiched between the ferromagneticterminal members 44 and 45. The ferromagnetic members have an openingfor allowing the passage of a fastener 46 having threaded ends, the nuts47 and 48 applied on to the ends keeping the stack of the ceramic magnetunits and the ferromagnetic members together. The ferromagnetic membershave their lower edges projecting beyond the ceramic magnet units, sothat only the pole surfaces of the ferromagnetic members come intocontact with the surface of the body 49 to be protected. Theferromagnetic members preferably have the same potential in thepotential series as the potential of the material of the body to becathodically protected.

The ferromagnetic members are electrically conductively interconnectedby the fastener 46. The ceramic magnet units have an opening 42 of sucha large diameter that the fastener is allowed to pass with ample play toprevent this member from closing the magnetic power current. In order toprevent such closure of the magnetic power current, the fastener 46 mayconsist of non-ferrous metal. According to the drawing, the magnetsystem comprises only two ceramic magnet units. If, however, the spacein the recess 37 is increased accordingly, the magnet system may beextended to a stack of more ceramic magnet units, each time withinsertion of a ferromagnetic member.

The terminal faces of the fastener 46 have threaded bores by means ofwhich the magnet system is mounted to the electrode 39. The recesses 37and 38 may be filed with plastic material which does not cover the polesurfaces of the ferromagnetic auxiliary members.

We claim:

1. A method of cathodically protecting a ferrous metal object againstcorrosion in an electrolyte, comprising the steps of providing a magnetof high dielectric ceramic material, mounting a protecting electrode onsaid magnet, and placing said magnet with the protecting electrodethereon on the object to be protected with the magnet directly againstthe object and in the electrolyte against which the object is to beprotected, and electrically connecting the electrode and the object tobe protected through a path outside said magnet, whereby the protectingelectrode is secured tightly through the magnet to the object to beprotected and the magnet also acts to electrically insulate theelectrode from the object to be protected.

2. A method as claimed in claim 1 in which the electrode is asacrificial electrode and the sacrificial electrode is electricallyconnected to the object to be protected through a simple resistancecircuit.

3. A method as claimed in claim 1 in which the connection of theprotecting electrode to the object be protected through a point outsideof the means is through a source of electric potential for impressing apotential on the electrode.

4. A method as claimed in claim 1 further comprising covering all of thesurfaces of said magnet other than the surface thereof which is to lieagainst the object to be protected with a synthetic electrolyteresistant material, and the step of placing the magnet on the object tobe protected comprises placing said uncovered surface of the magnet incontact with the object.

2. A method as claimed in claim 1 in which the electrode is asacrificial electrode and the sacrificial electrode is electricallyconnected to the object to be protected through a simple resistancecircuit.
 3. A method as claimed in claim 1 in which the connection ofthe protecting electrode to the object to be protected through a pointoutside of the means is through a source of electric potential forimpressing a potential on the electrode.
 4. A method as claimed in claim1 further comprising covering all of the surfaces of said magnet otherthan the surface thereof which is to lie against the object to beprotected with a synthetic electrolyte resistant material, and the stepof placing the magnet on the object to be protected comprises placingsaid uncovered surface of the magnet in contact with the object.